[AL phy]点解gravitational PE一定系负数?

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更新1:

re physics8801: 咁如果PE>0
咁咪要远过inifinity??

更新2:

re physics8801: F=负dU/Dx 其中U ""本身""系负数
定又可以系正
又可以负?

点解gravitational PE一定系负数? The reason is because of the choice of reference level for zero potential energy. It is conventionally adopted to use infinity as a reference level for zero potential energy. Since graviatational force is an attractive force
when an object falls from infinty (zero potential energy) towards the attracting object
e.g. the earth
its potental energy decreases
i.e. its potial energy value should be lower than zero. In mathematics
any value less than zero is negative. In other words
the potential energy bees more and more negative with respect to that at infinity when the falling object is nearer and nearer to the earth. 点解F=负dU/dx??? The physical significance for the negative sign is that the direction of the attractive force is to [decrease] the potential energy of the falling object. In mathematical term
the attractive force has a negative potential gradient. The force acts in a direction to decrease (i.e. -dU) the potential energy. This is
in fact
a property of attractive force. 但点解atom之间o既PE 又可以正又可以负呢???? Intermolecular force is attractive in nature. Thus the potential energy is neagtive. That is
the potential energy decreases with decrease of distance beeen the atoms. However
for very close distance beeen o atoms when the electrons of these atoms are overlapping one another
the force bees repulsive and the potential energy then increases with decrease of distance. energy系sacle
咁咩情况系负
咩情况系正数呢??? Notice that the sign affixed to energy indicates its magnitude (unlike a vector quantity where the sign indicates its direction)
and which shows whether the force involved is attractive (negative potential energy) or repulsive (positive potential energy).
负数 代表 attractive 正数 代表repulsive gravitational PE一定系负数 因为 一定系attractive ... 被地球吸... atom之间
可以系attrative or repulsive so
正又可以负.... energy系sacle
但系佢前面个正 or 负 唔系指direction.. 系指attractive

repulsive ‧ o‧●╔╦╗ ╔╦╗ ╔╦╗ ╔╦╗○‧o‧ ‧o ‧○╠希╣.╠望╣.╠帮╣.╠到╣你 啦~● ‧o‧ ‧ o‧●╚╩╝ ╚╩╝ ╚╩╝ ╚╩╝○ ‧o‧ ∴ ╭╯╰╮ ﹒．﹒‧∴﹒．

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